1. Technical Field
This invention generally pertains to communication of information between a source and a receiver. More particularly, it concerns the use of source distance information at the receiver to ensure maximum bandwidth of communication and avoid noise and interference from other sources operating over the same frequencies.
2. Brief Description of the Prior Art
In his classic paper titled “A mathematical theory of communication” (Bell System Technical Journal, vol. 27, pages 379–423, 623–656, 1948), Claude E Shannon defined the object of communication technology as enabling the transfer of information from a source to a receiver. A perfect receiver should be accordingly defined as one that could receive an arbitrary signal f(r, t) from a transmitter at a relative distance r without noise, distortion or interference from any other source. Such an ideal receiver is unachievable by Shannon's theory, but current technology is especially worse off with respect to the criterion of interference, because it splits the available physical bandwidth in some way to keep the signals from multiple sources separate all the way. The separation techniques include frequency division multiplexing (FDM) used in radio and television broadcast, wavelength division multiplexing (WDM) and mode separation in optical fibres, spread-spectrum encoding or code division multiple access (CDMA), or time division multiplexing (TDM) and its asynchronous variant, the Ethernet. Recent variations of this theme include blind signal processing as discussed in the book titled Adaptive Blind Signal and Image Processing (Wiley, 2002, authors A Cichocki and S Amari), which uses statistical analysis to cope with the distortion of the original separation parameters by the wireless channel, and autocorrelation matching, in which a “prefilter” is applied at the source, as described by R Liu, H Luo, L Song, B Hu and X Ling in their paper titled “Autocorrelation—a new differentiating domain for multiple access wireless communication”, in the Proceedings of ISCAS, 2002. All of these techniques effectively share the channel capacity corresponding to the usable physical bandwidth.
Lately, another such idea, using multiple transmitting and receiving antennae in parallel, is called space division multiplexing (SDM), as in the articles “Reduced complexity space division multiplexing receivers” by G Awater, A van Zelst and R van Nee in the Proceedings of IEEE Vehicular Technology Conference, May 2000, and “Channel Estimation and Signal Detection for Space Division Multiplexing in a MIMO-OFDM System”, by Y Ogawa, K Nishio, T Nishimura and T Ohgane in IEICE Transactions on Communications, Vol. E88-B, No. 1, January 2005. The usage is debatable, since it concerns merely using a larger antenna cross-section to achieve a correspondingly larger channel, i.e. there is no actual division of space whatsoever, even though the parallel antennae could be using complementary polarizations, which, if used to transmit different channels, would have qualified as a form of space division multiplexing. However, it would still be far ambitious than the objects and motivation of the present invention, as follows.
If only we could string separate cables or fibres between each receiver and its selected source, the sharing of the channel capacity would become unnecessary, and the entire capacity of the cable or fibre link would become available to each receiver and its respective source. It is desirable to have a similar capability for wireless technology, which is being steadily pushed into increasingly higher frequency bands for bandwidth as channels compete within the same frequency bands over the same physical space. The main challenges are directivity and range selection. The first is partly addressed by using high operating frequencies so that the wavelengths are comparable to or less than the receiver dimensions, and partly by phased array technology, which enables source direction selection without physically moving an antenna. There has been no practical solution for the second, although as range and angle are mutually complementary as physical dimensions of space, and on that basis, a similar, receiver-side technology could have intuitively thought possible.
The present invention is a solution based on a method for enabling a receiver of electromagnetic or other waves to determine the distance r to the source of the waves by modifying a receiver parameter, as described in copending application titled “Passive distance measurement using spectral phase gradients”, filed 2 Jul. 2004, No. 10/884,353, incorporated here in its entirety by reference. The method involves varying an instantaneous frequency selector ŵ at the receiver at a rate α, whereby frequency shifts δw become induced in the received waves in proportion to αr, so that r can be computed from α and δw. This method avoids round trip timing (RTT) and coherent phase reference requirements, but it depends on the phase distribution, which is already utilized in current technology. For example,                any kind of modulation as such involves a nonzero bandwidth spread, and frequency modulation (FM) especially relates to phase modulation (PM),        PM itself is also in use, for instance, in data modems as quadrature phase shift keying (QPSK), and for the encoding of colour in PAL and SECAM broadcast television formats, and        in any case, all signal processing, including the autocorrelation matching and the blind signal processing methods mentioned above, involve manipulation of the signal phase.It has not been obvious, therefore, that this method can be used for source selection without being impacted by modulation or signal processing, and without interfering with these operations. Moreover, in the presence of a modulating signal, a received carrier is no longer an almost pure sinusoid, as would appear to be assumed in the copending application, so that even the inferred distance r(ŵ) would vary significantly over the received modulated carrier bandwidth.        
It is in fact generally unobvious how any form of distance determination could help in signal selection or source isolation. An independent review of the copending application method observed, for example, that timing or coordinate information from the global positioning system (GPS) could be encoded in transmitted signals to enable source distance determination without RTT or coherent phase reference. While the method could have other applications, it would be specific to signals actually bearing the encoded coordinate or timing information and thus less than general. The encoded information would be generally available only after the signals are separated, and would be thus useless for the separation itself.
A hitherto unaddressed need exists, therefore, for a method that can separate signals from multiple sources, that does not interfere with the signal phase distribution or depend on the signal form or content, and would enable the entire physical bandwidth available from a source to be utilized for communication with only that source, without interfering with signals from another source. Such a separation is available for sources located at different directions from the receiver using phased array antennae as remarked, but not for sources along roughly the same direction and differing only in distance. It would be also desirable to have a method that can be applied over a large gamut of operating wavelengths, for example, from radio waves to ultraviolet frequencies, and even to acoustic waves. It would be additionally desirable for the method to be also useful for detecting the presence of multiple sources, i.e. of interference, so as to enable a receiver to lock on to and track a selected source.